1. Field
The present disclosure relates to a hyper-branched polymer, an electrode for a fuel cell including the hyper-branched polymer, an electrolyte membrane for a fuel cell including the hyper-branched polymer, and a fuel cell including at least one of the electrode and the electrolyte membrane.
2. Description of the Related Art
Fuel cells that include a polymer electrolyte membrane operate at relatively low temperatures and may be manufactured to be small in size. Thus, such fuel cells are expected to be used as energy sources in electric vehicles and in distributed generation systems. Perfluorocarbon sulfonic acid-based polymer membranes, such as NAFION® membranes (available from E.I. du Pont de Nemours and Company), are commonly used as polymer electrolyte membranes for fuel cells.
However, such polymer electrolyte membranes should be humidified in order to sufficiently conduct protons. In addition, to enhance cell system efficiencies, polymer electrolyte membranes should be operated at high temperatures, i.e., at least 100° C. However, the moisture in the polymer electrolyte membrane evaporates and is depleted at such temperatures, which reduces the effectiveness thereof.
To address such problems and/or other problems in the related art, non-humidified electrolyte membranes, which may operate at temperatures of at least 100° C., without humidification, have been developed. For example, a phosphoric acid doped polybenzimidazole non-humidified electrolyte membrane has been disclosed.
In addition, in cells that operate at low temperatures, such as the cells including a perfluorocarbon sulfonic acid-based polymer membrane, electrodes that include polytetrafluoroethylene (PTFE) as a waterproofing agent have been widely used to prevent gas diffusion in the electrodes due to formation of water produced as electricity is generated.
In addition, phosphoric acid fuel cells, which operate at temperatures in the range of from 150 to 200° C., include a liquid phosphoric acid electrolyte. However, the liquid phosphoric acid included in a large amount in the electrodes of the fuel cells interferes with gas diffusion in the electrodes. Therefore, an electrode catalyst layer that includes a polytetrafluoroethylene (PTFE) waterproofing agent has been used to prevent fine pores in the electrodes from being clogged by the phosphoric acid.
In addition, in fuel cells including a polybenzimidazole (PBI) electrolyte membrane, which uses a phosphoric acid as a non-humidified electrolyte, in order to reduce contact between electrodes and the electrolyte membrane, a method of impregnating the electrodes with a liquid phosphoric acid has been used, and a method of increasing a loading amount of metal catalysts has been used. However, such fuel cells do not exhibit improved properties.
In addition, when a phosphoric acid-doped solid polymer electrolyte is used, and air is supplied to the cathode, an activation time thereof is about 1 week, even when an optimized electrode composition is used. Although the performance of the solid polymer electrolyte may be improved and an activation time may be shortened by supplying oxygen to the anode instead of air, such supplying of oxygen is undesirable for commercial use. In addition, a homogeneous polymer electrolyte membrane using the PBI is not satisfactory in terms of mechanical characteristics, chemical stability, or capability of retaining a phosphoric acid. Thus, there is a demand for further improvement.